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Molecular Manipulation Improved Structure of Polymers

EVANSTON, Ill. - A research team led by Northwestern University
materials scientist Samuel I.
Stupp has developed a novel method to improve polymers that could impact not
only the plastics
industry, but fields as diverse as optical communications, medicine and
nanotechnology.

This new method improves polymers by changing the actual organization of the
macromolecules
using small molecules as additives, rather than changing a polymer's
chemical structure as
catalysts do.

Stupp, Board of Trustees Professor of Materials Science, Chemistry and
Medicine, will present a
paper outlining these findings at the 219th American Chemical Society
National Meeting in San
Francisco at 1 p.m., U.S. Pacific Time, Thursday, March 30.

"Companies are interested in improving mechanical, thermal, transport, flow
and other properties of
polymers," said Stupp. "To achieve this, they've focused most of their
research dollars on the
chemistry of catalysts used to make polymers, but this will have limited
results. We are using
molecular self-assembly to physically change polymers - a completely
different direction that
holds a great deal of promise."

The researchers have discovered a system of molecules that when dissolved in
a liquid monomer,
such as styrene, form nanoribbons, reminiscent of DNA strands. Molecules
freeze around the ribbons in
an orderly fashion, completely changing the physical nature of the liquid
monomer and creating a
gel with a blue-violet hue, which appears like a liquid crystal when viewed
in a microscope.
Strikingly, the structural changes are retained when the liquid monomer is
polymerized into a solid,
such as polystyrene.

Today's polystyrene - used for such common items as food packaging, compact
disc jewel boxes,
appliances, television cabinets and toys - is inexpensive but has limited
toughness. "When
manufacturers need increased toughness and stiffness or other special
properties, they have to turn to
more expensive plastic, such as engineering plastics or liquid crystal
polymers," said Stupp. His
modified version holds the promise of sophisticated properties at an
inexpensive price.

One of the advantages of Stupp's method is the orientation of polymer
molecules in the material by a
nano-sized and stiff scaffold, formed by self-assembling molecules, in the
material's interior. It is
well known that polymeric materials are strong along the covalent axis of
their molecules because a
great deal of energy is required to break covalent bonds, says Stupp. The
nanoribbons orient easily
along a desired direction and drag the polymer chains around them.
Therefore, the method has
enormous potential for producing extremely strong polymers without requiring
the complex equipment
currently necessary to make ultra-strong fibers.

The researchers found that when minute amounts of designed molecules, which
they call dendron
rodcoils, are dissolved in monomers, the molecules interact with one
another, forming weak bonds and
assembling into ribbon-like structures. These tiny ribbons - hundreds of
nanometers long but only 10
nanometers wide and a few nanometers thick - are scattered throughout the
monomer. (By contrast,
a human hair is approximately 10,000 nanometers wide.) The final solid
polymer contains 108 meters
of nanoribbon per cm3, but the ribbons are so thin that they account for
only one percent or less of the
weight of the entire material.

"The critical next step was to see how the polymerization process, which
requires heating the
styrene, would affect the self-assembled ribbons," said Stupp. "Would solid
polystyrene, the
material used in thousands of everyday items, exhibit the same structural
properties as the liquid
styrene? The answer was a resounding yes."

In the presence of the ribbons, polymer chains line up neatly alongside
them. Without the rigid
ribbons to guide them, polymer molecules are amorphous coils, resembling a
jumbled pile of cooked
spaghetti, with chains heading in all directions.

Another advantage of Stupp's method is that the presence of the ribbons also
changes polystyrene's
optical properties dramatically. The polystyrene becomes strongly
birefringent, a property that
could be exploited to move light in specific directions. The modified
polystyrene also can reflect and
transmit certain wavelengths of light. These optical properties are mediated
by the nanoribbons, so
the material used to make cheap plastic parts also could become a material
for advanced photonics.

"Molecular self-assembly has changed the structure of polystyrene
completely," said Stupp. "And it
doesn't involve searching for new catalysts."

Stupp's team next plans to investigate the mechanical and flow properties in
the modified
polystyrene, as well as the use of the self-assembled ribbons in other
polymers. "We can modify
many different materials, and we already know we can modify monomers that
polymerize into
rubbers with this method," said Stupp. "But the properties exhibited by each
may differ." They also
will look at using the ribbons in biology for medical purposes, to create
structures that direct cells to
travel in a certain direction, for example.

The research was funded by the U.S. Army Research Office, the National
Science Foundation, the
Department of Energy and the Office of Naval Research.

Stupp's research collaborators are postdoctoral researcher Eugene R. Zubarev
and graduate student
Martin U. Pralle, both of the University of Illinois at Urbana-Champaign,
and graduate student Eli
D. Sone of Northwestern.